Bevel Gear Set-up
Bevel Gear Set-up
(OP)
Power is transmitted to my Ariens mower through a gearbox mounted on the mower deck. Ariens has been using this design for at least 25 years and before that it was used by New Holland. The mower is no longer in production but Ariens will build one for you from parts which is what I just did. Unfortunately, the new gearbox had excessive endplay on the output shaft (0.028" vs spec of 0.003/0.008")which could not be fixed. Usually there are shims which can be removed to reduce endplay but this unit had excessive endplay with no shims. The replacement gearbox was the same. My dealer did a non-Ariens approved modification to reduce endplay (something he has done many times over the years) but now the box has a high pitch whine at full speed and an intermittent scraping noise at half-speed.
Ariens Engineering Department has set-up another gearbox and is sending it to the dealer. They are not forthcoming about what caused the original problem other than saying it was due to an accumulation of tolerances. This doesn't strike me as plausible.
The box design is somewhat crude. It is aluminum diecast with the bearing races for the vertical output shaft located by non-machined shoulders in the diecast. The horizontal input shaft bearing races on either side of the input gear are adjustable to set input shaft endplay and gear backlash. The gears are straight bevel and have no markings to set mounting distance or to identify mating teeth. The service manual does not specify gear mounting distance but says backlash should be 0.005/0.015".
Unfortunately, I think all of the new Holland/Ariens engineers who know about this box have retired and Ariens says they will obsolete the part instead of investing much effort to find the problem. I am expecting the third box will not be much better than the first two.
Can someone give me some suggestions on how to set-up this box? Or, where can I find a person or shop that can set this box up for me? Thanks for your help.
PS I will try to post a drawing of the gearbox although I think you all can probably visualize it.
Ariens Engineering Department has set-up another gearbox and is sending it to the dealer. They are not forthcoming about what caused the original problem other than saying it was due to an accumulation of tolerances. This doesn't strike me as plausible.
The box design is somewhat crude. It is aluminum diecast with the bearing races for the vertical output shaft located by non-machined shoulders in the diecast. The horizontal input shaft bearing races on either side of the input gear are adjustable to set input shaft endplay and gear backlash. The gears are straight bevel and have no markings to set mounting distance or to identify mating teeth. The service manual does not specify gear mounting distance but says backlash should be 0.005/0.015".
Unfortunately, I think all of the new Holland/Ariens engineers who know about this box have retired and Ariens says they will obsolete the part instead of investing much effort to find the problem. I am expecting the third box will not be much better than the first two.
Can someone give me some suggestions on how to set-up this box? Or, where can I find a person or shop that can set this box up for me? Thanks for your help.
PS I will try to post a drawing of the gearbox although I think you all can probably visualize it.





RE: Bevel Gear Set-up
RE: Bevel Gear Set-up
I'm guessing your dealer is breaking in a new tech on the old stuff, and he set yours up just a little tight. Give him a chance to rectify that.
If the gears' axial position were settable with screws, e.g. with threaded collars around the bearings, I'd start with the gears lightly pressed into each other (zero lash), and back off the adjusters equally until there was maximum backlash, then run in the complementary adjusters until there was minimum backlash with the shafts pressed toward each other. Now, all you have to do is figure out how to do something equivalent with shims.
Mike Halloran
Pembroke Pines, FL, USA
RE: Bevel Gear Set-up
The person who modified the gearbox has been working on these boxes for 25 years and I have known him almost that long. He added custom shims between the case shoulder and lower bearing race on the output shaft. This allowed him to then come back in with stock shims between the bearing and output shaft shoulder as shown on the drawing. This modification should not affect gear performance. The box is set-up loose as a goose with shaft endplays at 0.008" and backlash at 0.015". The original box had 0.030" backlash and 0.028" output shaft endplay. The box ran quietly with this excessive play but I don't think it would have lasted very long.
Having said all of that, how do you bring the gears together if you don't know the mounting distances? Do I just bring the back angles flush? Also, what do you mean by complementary adjusters? Thanks!
RE: Bevel Gear Set-up
I think basically you have to start with zero lash to get the gears in the proper relative position, then move them away from zero lash to the minimum backlash setting, set the shims or collars or whatever so the gears can't move closer than that, then use the remaining shims collars whatever to not allow them to exceed the maximum backlash.
Of course, that nice idealized sequence doesn't fit nicely with the particular gearbox design, which makes my head hurt.
Mike Halloran
Pembroke Pines, FL, USA
RE: Bevel Gear Set-up
Then the output shaft endplay is set by the shims at its lower end.
Then the input gear is positioned, by the side cover gasket thickness, then the input shaft endplay is set with the adjustable plug on the input shaft.
That's just me looking at the assembly drawing. The approved procedure may be considerably different. As you said, the factory engineers may not know the _real_ factory procedure, that probably retired or died with someone.
Mike Halloran
Pembroke Pines, FL, USA
RE: Bevel Gear Set-up
There are two adjustments you must make for each shaft. One for the correct preload on the tapered roller bearing pairs, and one for the correct mounting distance of the gears.
Looking at your picture, your adjustment procedure should start by setting the correct bearing clearance on the vertical shaft by selecting the correct washer thickness (ie. .005, .015, .025 or .032 inch). There are no provisions for setting the gear mounting distance on this shaft. Next you should set the correct backlash on the horizontal gear shaft by selectively fitting the correct thickness combination of gaskets on the right side, and then setting the correct bearing clearance by adjusting the plug on the left side.
As for the recommended RT shaft end play of .003 to .008 inches, that seems a bit excessive for the tapered bearing arrangement shown. A backlash of .005 to .008 inch is probably OK for this class of gear, but you should also do a contact pattern check to make sure the gear mesh contact is fairly centered on the tooth, heel-to-toe.
After making your adjustments, make sure all your fasteners are properly torqued and locked.
Hope that helps.
Terry
RE: Bevel Gear Set-up
Terry, you are also 100% correct in saying that the output shaft endplay is established first. The specified backlash is 0.005-0.015". The first two gearboxes came from the factory close to 0.030". From what I have read, determining the proper contact pattern is more of an art than science. Do you think someone like myself can do it properly?
Mike, why does this box design make your head hurt? The main problem I see is that the diecast case is not machined to make sure the shafts are perpendicular. The only machining is cutting threads for the adjustment plug.
PS If I would have know how much fun gear design is, I would have been a mechanical engineer!
RE: Bevel Gear Set-up
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RE: Bevel Gear Set-up
- It requires adjustments during assembly.
- The adjustments are not obvious, and interact with each other, and are laborious to effect.
- It appears that use of the factory approved assembly procedure does not produce an assembly that meets the factory approved specs. This has apparently been true for decades, and nobody at the factory lifted a finger to do anything about it, thus leaving the dealers to homebrew solutions, and to absorb labor costs that they shouldn't have incurred.
- It was apparently "value engineered" by some dumbass bean counter who concentrated on part cost, hence the cross-pins instead of shoulders on the input shaft.
Mike Halloran
Pembroke Pines, FL, USA
RE: Bevel Gear Set-up
I believe the biggest problem I may have is that the input & output shafts may not be perpendicular and may not be in the same plane. I hope I can determine this from the tooth contact pattern.
RE: Bevel Gear Set-up
- toothpaste
- Dykem or permanent Magic Marker
- Loctite PST
You won't get quite the same pattern as a hypoid will produce, but the pattern's position relative to the tip and root will tell you something about the adjustments you can make.
If the housing bores are misaligned, there's not much you can do. .... but because it's a die casting, _all_ of them would be misaligned the same way, so it's not likely to have gone uncorrected.
Mike Halloran
Pembroke Pines, FL, USA
RE: Bevel Gear Set-up
I found some contact patterns in the Arrow Gear catalogue for shafts which are not perpendicular or not in the same plane. I do not know how well they apply to straight bevel gears but I will give it a try.
I can't tell you how much I appreciate your help.
RE: Bevel Gear Set-up
RE: Bevel Gear Set-up
http://www.youtube.com/watch?v=Zi9PBZCgw-0
I got the new box from Ariens. It has more endplay than spec but it is much better than the earlier ones. It is also much quieter.
My local NAPA store carries Permatex Prussian Blue. I tried to look at the contact pattern by using the grease in the box. It doesn't work that well but I can see two things: (1) the pattern is very high and (2) I have hard lines. No question that the gears are not mating properly. I will get everything cleaned-up and use the Prussian blue.
Regarding nomenclature, Arrow and others refer to the gear set as "pinion and gear" or "pinion and ring gear". In my application, would the horizontal input gear be the "gear" and the vertical output gear be the "pinion"?
RE: Bevel Gear Set-up
Mike Halloran
Pembroke Pines, FL, USA
RE: Bevel Gear Set-up
If you have access to some basic inspection equipment (surface plate, blocks, dial indicator and height gauge) you can check to see if the shafts/housing bores are coplanar and perpendicular. These problems are not things that can be fixed by shimming. Gear mounting distances and bearing clearances can be fixed with the appropriate shims.
As I mentioned previously, the .030 inch shaft end play seems way to large for your gearbox's tapered roller bearing sets. If your gearbox housing is diecast aluminum and your shafts are steel, then that shaft's axial clearance should be set close to zero at room temp, since the clearance will only increase at operating temps. The tapered roller bearings in your drawing look to have a contact angle around 20deg, which would equate to a radial runout of about .010 inch. And that amount of radial runout would not normally be acceptable at the gear mesh point.
Hope that helps.
Terry
RE: Bevel Gear Set-up
I disassembled the box last night. All of the red grease around the tapered bearings was black from what I assume to be Al particles. The bearing races for the vertical output shaft just fell out of the case. Don't know if the vibration caused this or if they never had a proper interference fit. Loctite has a product made specifically for cylinders (as opposed to threads) which I will use when I reassemble. This box has about 3-4 hours run time.
RE: Bevel Gear Set-up
The Loctite I mentioned was #609 or 620.
RE: Bevel Gear Set-up
Tore the box down and cleaned everything up. Other than the output shaft bearing races being loose, it didn't look too bad although it shouldn't with just a few hours. I did a couple contact patterns at 0.006 & 0.013" BL. Horizontal shaft input gear has 22 teeth and vertical shaft output gear has 19 teeth. Output gear was hitting very low and teeth 1-8 rotated smoothly but 9-19 were binding. No wonder she was howling. Now the BIG experiment. Added 0.020" under the output gear. WOW! What a difference. Had to add more shims to the side cover to reopen BL. Got it to 0.010" with the shims I had. Since the spec is .005-0.015", I'll probably leave it there. With this set-up, the contact pattern looks good and the gears turn very smooth. I might raise the output gear a little more to see where the optimum point is, but I must be getting pretty close.
Of course, I won't know for sure until I put it under load, but the difference in the no load mode is dramatic.
Thanks again for your interest and help.
RE: Bevel Gear Set-up
Do let us know how well it works under load, too.
Mike Halloran
Pembroke Pines, FL, USA
RE: Bevel Gear Set-up
"As I mentioned previously, the .030 inch shaft end play seems way to large for your gearbox's tapered roller bearing sets. If your gearbox housing is diecast aluminum and your shafts are steel, then that shaft's axial clearance should be set close to zero at room temp, since the clearance will only increase at operating temps. The tapered roller bearings in your drawing look to have a contact angle around 20deg, which would equate to a radial runout of about .010 inch. And that amount of radial runout would not normally be acceptable at the gear mesh point."
Ok. Al expands 2x steel but the Al case will be air cooling while the steel shafts will not. It may be that this relative cooling will largely offset Al's higher thermal expansion. Or, maybe the steel shafts will actually expand more than the Al case. I am not sure how to allow for thermal expansion. Also, on the input shaft, differential shaft expansion will decrease endplay where as on the output shaft, differential shaft expansion will increase endplay. I want to get this box right but I don't want to outsmart myself. Because I don't know actual operating temperatures of the shafts and case, I am inclined to set both shafts at close to zero even though the spec is 0.003-0.008". Does this seem reasonable? Thanks.
RE: Bevel Gear Set-up
No the bearings or gears will seize, must have the mfg end play
or there will be to much friction & heat.
Mfgenggear
RE: Bevel Gear Set-up
"No the bearings or gears will seize, must have the mfg end play or there will be to much friction & heat."
I still disagree. Based on what the sketch shows, .028" end play in a gear shaft mounted in tapered roller bearings would produce excessive runout at the gear pitch line. And apparently, that's what seems to be occurring. Tapered roller bearing pairs are designed to run with little or no axial play. If there is excessive endplay/radial clearance, the tapered roller ends will lose the aligning contact with the inner race shoulders and will skew and skid. This will result in excessive bearing heat generation and scoring failure.
As for clearance changes due to CTE mismatch between shaft and housing, there's no way it will be anywhere near .030 inch. Looking at the sketch, I'd guess that the bearing spacing is about 6 inches max. With a CTE mismatch between the aluminum housing and steel shaft of around 6ppm/degF, that gearshaft would need to get almost 850degF hotter than the housing to take up .028 axial clearance.
Finally, if one shaft is OK with the specified .003 to .008 inch endplay, using a similar bearing arrangement and subjected to similar operating loads and speeds, then why wouldn't the other shaft also be OK?
Terry
A quick test can be done by assembling the gearbox with your desired clearances and heat soaking it in an oven to whatever temperature you think it will see. Take it out and check to see if it still turns freely.
RE: Bevel Gear Set-up
I can do the temp test as you suggested, but it will not account for temperature gradients that will exist in normal operation. Calculating relative thermal expansion is straightforward if you assume all components are at the same temperature. My concern is that the Al box will stay cool relative to the internal steel shafts. This temperature may or may not be enough to offset Al's 2x thermal expansion.
I was wondering if there are rules of thumb or other general experience which would indicate the relative operating temperature of a gearbox case compared to its internal moving parts?
RE: Bevel Gear Set-up
Maybe they will subcontract the assembly operation to you.
It might bring in a little beer money.
Mike Halloran
Pembroke Pines, FL, USA
RE: Bevel Gear Set-up
RE: Bevel Gear Set-up
From there, you need to set the bearings so that the backlash cannot be forced to zero, and then to provide just enough axial play to not jam the bearings, for which the 3 to 8 mils sounds just fine.
The only remaining question is, how many can you do in an hour? That will help you set a price for outsourcing. I am absolutely not kidding about that, either. Ariens has clearly lost whatever expertise they once had, and whatever (s)he was doing was not documented well enough so the survivors could do it.
Mike Halloran
Pembroke Pines, FL, USA
RE: Bevel Gear Set-up
The Ariens story is sad. I am sure that when this box was first designed in the early 70's, component parts were speced to achieve good gear mesh on a reproducible basis. Over time, suppliers changed and specs drifted to the point where a couple shims were required here and there to make things right but bo one is assigned to perform the checks and install the shims. Gearbox assembly has been subbed out to people who just put it together. Heck, Ariens only QC test is to run it and listen for noise. If it is sufficiently quiet (whatever that means) they let it go. No one checks or cares if it has 30 thou BL or 30 thou endplay.
My old box lasted 18 years. And, if I knew then what I know now, I could have given it a tune-up every 4 or 5 years and it probably would have lasted forever.
I have now officially buried the differential thermal expansion bogeyman!
RE: Bevel Gear Set-up
since this is not a precision assembly
I mean it is not precision machined to the spec's that we are used too. there fore the backlash & end play is better to be more loose then tighter.
Probably very loose tolerance gears. guess !
& the housing according to the OP is not machined.
so more end play & back lash is required.
I would say that the assembly of the bevels & shafts
are not square & there is probably misalignment between the
the mounting surfaces.
To make this assembly better, bore out the holes (mounting surfaces) & press a steel or bronze bushing into the holes
& re machine to a better precision.
but lots cost to do all this work, so i recommend live with a little more end play.
I agree .028 end play is to much .
.012 end play should be fine.
my two cents
Take Care
Mfgenggear
RE: Bevel Gear Set-up
I agree that there's likely geometry errors. There's only one reason gears howl- the gear teeth moving in/out of mesh are fighting each other. The numbers of teeth (19/22) on these gears make the problem worse with a low contact ratio.
Thoroughly enjoyed the discussion though.
Terry
RE: Bevel Gear Set-up
The only remaining question I have is how to prevent anymore bearing race spinout on the input shaft. I don't want to Loctite them because I may need to move them someday to adjust BL or endplay. I may drill/tap the case and use a setscrew. Of course, the races may not spin now that the gears are properly aligned.
RE: Bevel Gear Set-up
Mike Halloran
Pembroke Pines, FL, USA
RE: Bevel Gear Set-up
Is a 15 minute dyno run sufficient to determine if the gears have been properly set-up? The latest box they sent me was relatively quiet initially but was howling badly after 3 hours.
RE: Bevel Gear Set-up
They are apparently assuming that a 'bad' box will frag itself within 15 minutes, no instrumentation needed.
Sounds like an MBA is running things there.
Mike Halloran
Pembroke Pines, FL, USA
RE: Bevel Gear Set-up
PS Hope to have my old box, which I believe is set-up properly, running by the end of the week.
RE: Bevel Gear Set-up
If the gears _are_ set up correctly, a minute or so with a varying load and an accelerometer or a microphone should be sufficient to verify it. Lacking a computer controlled dyno, it might take a bit longer; 15 minutes should be more than sufficient even with primitive tools.
Mike Halloran
Pembroke Pines, FL, USA
RE: Bevel Gear Set-up
If you got a smoother operating bevel gear set by reducing the mounting distance of one of the gears, then what you likely had was an issue with contact ratio and index errors. This would be especially true with the low numbers of teeth and low precision your gears have. They likely need some tip mods or face profile mods.
A 15 minute dyno run is more than sufficient for acceptance testing of your gearbox. If it is going to fail in scoring, it will happen in a minute or two.
Don't be too concerned about backlash, it's not usually an issue with involute geometry gears.
Regards,
Terry
RE: Bevel Gear Set-up
RE: Bevel Gear Set-up
I don't know what to make of the contact pattern. The output (overhung) gear had a nice smooth (no lines or scoring) pattern in the center of the tooth. However, the input (straddle-mounted) gear had a high pattern concentrated at the toe. There are definite lines of contact which seem to be the same for all teeth. Also, the leading edge (drive side of tooth) of the top land is rounded over to the point where you can catch your fingernail on the ridge when you drag it across the top land perpendicular to the face width. It looks like the input gear is taking a beating whereas the output gear looks good.
Can you conclude anything from this rough verbal description?
RE: Bevel Gear Set-up
Nice job with the gearbox. I think it's great seeing someone with limited knowledge of gears take a structured approach to gain an understanding of a problem.
Per your latest "test report", anytime you have displaced metal (smearing or galling) on your gear flanks, it's due to contact pressures in excess of what the material can take. The smearing may be the result of excessive torque loads, load concentrations due to misalignment or geometry errors, loss of mechanical properties due to overheating, or low mechanical properties due to a manufacturing defect.
Getting a specific answer would likely entail more test work than you want to undertake. Maybe when you're done though, you can get Ariens to hire you as a consultant to redesign their gearbox.
Regards,
Terry